In cellular wireless communications systems a limited range of resources are reused in different, spaced apart, cells. The resources vary according to the type of system, but are generally frequency channels, time slots on a bearer channel, spreading codes or combinations of these. Cells may be subdivided into sectors, with each sector being served by one or more beams formed by directional, higher gain, antennas. The directional antennas increase performance in the uplink and downlink directions by reducing interference, for example, and also help to increase capacity of the overall system as the resources allocated to one beam or sector can be reused in other beams or sectors. Each beam may use a sub-set of the overall resources of the cell or resources may be reused in different beams within the same cell.
One of the problems which can arise in cellular systems is that the total traffic demand of the terminals in a cell, sector or beam poorly matches the capacity of that cell, sector or beam. While system operators attempt to provision sufficient resources to meet the expected demand, there can be periods when a cell, sector or beam becomes overloaded to the extent that it cannot provide a service to a new terminal. Alternatively, providing service to a new terminal may seriously degrade the amount of resources available to be shared among the existing terminals, thus degrading their service level. A cell may become overloaded as a result of an event which causes a ‘hot spot’ of terminal activity in a particular localised area. The division of cells into sectors increases the likelihood of uneven loading and the division of sectors into beams further increases the likelihood of uneven loading. Averaging the traffic load over a larger area, through the use of a larger cell, tends to reduce the unevenness in load between different cells while reducing the area of the cell, dividing a cell into sectors or dividing a sector into beams gives rise to an increased variability in load in any one cell, sector or beam. Services which use a larger proportion of the resources, such as high data rate multimedia services, result in a lower number of users being supported and this also leads to a greater variability in load from cell to cell, sector to sector, beam to beam, or time to time in a given cell, sector or beam.
One known way of addressing this problem is to vary the effective width of a sector or beam if a neighbouring sector or beam is known to be overloaded. In this way, the resources of one sector can be used to supplement those of the overloaded sector. While this can more evenly match the load to the available capacity of the base station, it requires a more complicated and expensive antenna arrangement and control system at the base station.
In systems employing adaptive modulation and coding (AMC) combined with equal throughput scheduling (EQT) a further problem arises that cannot easily be addressed by adapting the beam shape provided by the base station. In such systems, terminals located in areas where the received signal strength, or signal to interference plus noise ratio, in the uplink or downlink directions is badly affected by propagation effects are allocated an increased share of the available resources. Although all terminals now receive an equal level of service this technique distributes a disproportionately large share of the resources to the affected terminals and results in a reduction in the aggregate capacity of the cell, sector or beam. Such badly located terminals are not often conveniently positioned to enable support form an adjacent cell, sector or beam and even when they are, the amount of resources required from the adjacent sector or cell will often be equally disproportionate.
Accordingly, the present invention seeks to improve service to terminals in cellular systems.